/Engine/Source/Runtime/Engine/Classes/EditorFramework/AssetImportData.h
Add a filename at the specific index. Will update the imported timespan and MD5. It will also add in between with empty filenames
void AddFileName ( const [FString](API\Runtime\Core\Containers\FString) & InPath, int32 Index, [FString](API\Runtime\Core\Containers\FString) SourceFileLabel )